Automatic frequency control system



B; TREVOR AUTOMATIC FREQUENCY CONTROL SYSTEM Filed April 20, 1939 May13, 1941.

Patented May 13, 1941 2,241,937 AUTOMATIC FREQUENCY CONTROL SYSTEMBertram Trevor, Riverhead, N.

Y., assignor to Radio Corporation of America, a corporation of DelawareApplication April 20, 1939, Serial No. 268,871

3 Claims.

This invention relates to the art of automatically controlling thefrequency of the first beating oscillator in a superheterodyne receiverin order to keep the signal accurately tuned to the mid-frequency of theintermediate frequency pass band.

The method to be described uses certain well known features togetherwith novel additions and is particularly adapted for use with ultra highfrequency receivers where the electronic method of frequency controldoes not give satisfactory results. The principle is perfectly welladapted to low frequency as well as high frequency receivers.

Figure 1 shows the essential features of my frequency control system ofthe present application; while Figure 2 illustrates a modification ofthe output circuit of tubes I4 and iii of Figure 1.

Referring to the drawing: wave energy which may be modulated by signalsis amplified in 4 and reduced in frequency by heating with oscillationsfrom local oscillator 6. The wave energy of reduced frequency which maybe an intermediate frequency is amplified also in 4 and is fed to theinput of tube T1 the output of which drives the well known networkC1L1MCLaC2. The inductance Lo of this network feeds the wave energy setup in L2 to two rectifiers T2 and T3. The cathodes of the rectifiers T2and T3 are shunted by condensers C4, resistors 11 and resistors T2.Resistor r3 connects the rectifiers to the control grids Ill and I2 of apair of coupling and amplifying tubes l4 and IS. The network C1L1MCL2C2,when properly adjusted, has the property of giving equal and oppositerectified two diode load resistors 11 when the input frequency isexactly at mid-band in the intermediate frequency amplifier. Thiscondition hereafter referred to as, in tune" or balanced condition,gives a zero potential difference between the points Di and D2. The loadcircuit of the two diodes T2 and Ta is seen to be similar to that of abridge. The two resistors m in combination with the ground connectionbetween them serve to maintain the mid-point potential between D1 and Dzat ground potential regardless of the signal level at the input of T1.The direct current voltage appearing between D1 and D2 rises sharply asthe input frequency to T1 varies either side of balance and the polarityof this voltage is reversed as the signal frequency passes from one sideof balance to the other. This voltage is used to control the two directcurrent amplifier tubes l4 and Ni.

Turning again to the frequency discriminating currents through thecircuit r: and C5 serve as radio frequency filters, capacitors C4 serveas radio frequency by-passes, L3 is a radio frequency choke to returnthe rectified current from the diodes T2 and T3 to the mid-point of L2,and n. is the self-biasing resistor for the tube T1.

The tubes [4 and 16 comprise a push-pull direct current amplifier stagearranged to drive two motor control relays 20 and 22. It will beobserved that both relays are connected across the output of the directcurrent amplifier, that is between the plates l1 and H! of tubes l4 andIS with diode rectifiers 24 and 26 in series with each relay winding.These diode rectifiers may be separate systems or may be included in asingle envelope as is the rectifier designated in the trade as type 6H6.The polarity of the two diodes 24 and 26 is reversed thus allowing onlyone relay to operate at a time as the polarity of the voltage differencebetween the two amplifier plates l1 and I9 reverses with change of thesignal frequency. Under the balanced condition there will be no voltagedifference between these two plates and consequently, neither relay willdraw current.

In this amplifier circuit, the two resistors 30 and 32 in series withthe control grids of the amplifier tubes serves to eliminate anytendency for spurious oscillations to develop. The potentiometerresistance 34 connected between the screen grids 35 and 31 allows theamplifier to be balanced (zero voltage between plates) when there existsno voltage difference between the control grids l0 and 12. The switch 42between the control grids l0 and i2 is provided to connect these twogrids together in order that the balance potentiometer 34 may beproperly adjusted. This procedure then assures that there will be nocurrent through either of the relays 20 and 22 when the signal is atmid-band frequency which corresponds to the condition of no voltagedifference between points D1 and D2.

The two relays 20 and 22 in combination perform the function of a singlepole double-throw switch with a definite off position under balancedconditions. The use of a single polarized relay connected across thedirect current amplifier output has been used previously but this hasthe disadvantage of sticking contacts when a relay is used having aneutral off position. The relays shown at 2 and 22 can be the ordinarytelephone type giving very positive make and break contacts with notendency to stick. Furthermore, such relays are inexpensive and rugged.

The terminals M1M0M2 of the relays are connected to the windings of areversible motor 40. The reversible motor may be coupled throughsuitable reduction gears 50 either direct to the main tuning control ofthe oscillator 6 or to a Vernier condenser, or to a Vernier of any sortwhich produces a frequency change of the controlled oscillator. In thearrangement shown the reduction gearing 50 drives a shaft 52 connectedto a conductive plate 5 capacitively coupled to the inner and outermembers of a coaxial low loss oscillator circuit controlling thefrequency of the oscillator 6 supplying heterodyning oscillations to thefirst detector in 4. It is, of course, necessary that the motorterminals be polarized properly in order that the system described willbring the signal toward mid-band rather than away from it.

The neon lamp, i9, is provided in order that an operator can easily tellwhen the tuning motor is running. The lamp lights when the tuning motorruns in either direction. There are two spark suppressor circuits E2 and14, one across each relay contact, consisting of a .1 f. capacitor inseries with 100 ohms of resistance.

The advantages of the system described are:

1. When the signal is properly tuned to mid- V band neither of therelays draws current and the control motor is inoperative.

2. The system is not critical to signal level for normal operation.

3. In case of signal failure the control motor is rendered inoperativeand the receiver tuning controls remain fixed until the signal againappears.

4. Thesystem will pick up the signal over a band considerably wider thanthe of the receiver if automatic gain control is used to bring thesignal up to a working level to the input of the tube T1, even thoughthe signal frequency may be outside of the of the receiver.

5. The working parts of the system are simple and rugged givingtrouble-free operation.

6. The accuracy of tuning can be made very great by proper choice ofelements in the network C1L1MCL2C2 It is to be understood that thesystem described need not be applied only to the first oscillator in adouble detection receiver but may be applied to the first and/or secondoscillator of a triple detection receiver.

In a modification I make use of a single zero center meter 55 to takethe place of the two meters 23 and 25 in the diode circuit shown. Inthis modification illustrated in Figure 2 one terminal of the singlemeter 55 is connected to one of the output tube plates, say l1, whilethe other meter terminal is connected to the plate 24 and cathode 26 ofthe diodes remote from the coils of relays 20 and 22. The relay coilconnection remains unchanged.

What is claimed is:

1. In a system for maintaining the frequency of wave energysubstantially constant, wave energy amplifying means including a tunableelement which determines the frequency of the wave energy amplified, apair of rectifier tubes having input and output electrodes, a frequencydiscriminating circuit connected to the input electrodes of saidrectifiers, impedances connecting the output electrodes. of saidrectifiers in a balanced differential circuit, means for impressing saidwave energy on said frequency discrimina ing circuit, wherebysubstantially equal diiferential voltages are produced in saidimpedances by rectified Wave energy when said Wave energy imband width upressed on said frequency discriminating circuit is of the proper meanfrequency and unequal differential voltages are produced in saidimpedances when the frequency of said wave energy deviates from saidproper frequency, a pair of electron discharge amplifier tubes havinginput and output electrodes, meam coupling said input electrodes to saidimpedances, a pair of uni-lateral devices connected in parallel, saiduni-lateral devices being conductive in different direction, meanscoupling the said parallel devices to the output electrodes of saidamplifier tubes, driving means connected with said tunable element formoving said element in one direction when direct current flows throughone of said devices and in the other direction when direct current flowsthrough the other of said devices, and means actuated by current flow insaid uni-lateral devices for controlling the operation of said drivingmeans.

2. In a system for maintaining the frequency of Wave energysubstantially constant, wave energy amplifying means including a tunableelement which determines the frequency of the wave energy amplified, apair of rectifier tubes having input and output electrodes, a frequencydiscriminating circuit connected to the input electrodes of saidrectifiers, impedances connecting the output electrodes of saidrectifiers in a balanced differential circuit, means for impressing saidwave energy on said frequency discriminating circuit, wherebysubstantially equal differential voltages are produced in saidimpedances by r ctified wave energy when said wave energy impressed onsaid frequency discriminating circuit is of the proper mean frequencyand unequal differential voltages are produced in said impedances whenthe frequency of said wave energy deviates from said proper frequency, apair of electron discharge amplifier tubes having input and outputelectrodes, means coupling said input electrodes differentially to saidimpedances, additional impedances in a circuit connected to the outputelectrodes of said amplifier tubes, means for balancing said last namedcircuit whereby the voltages insaid additional impedances aresubstantially balanced when the Wave energy impressed on said frequencydiscriminator circuit is of the proper mean frequency, a pair ofunilateral devices connected in parallel, said unilateral devices beingconductive in different direction, means connecting the said parallelunilateral devices to said last named impedances, driving meansconnected with said tunable element for moving said element in onedirection when direct current flows through one of said devices and inthe other direction when direct current flows through the other of saiddevices, and means actuated by current flow in said uni-lateral devicesfor controlling the operation of said driving means.

3. In a system for maintaining the frequency of wave energysubstantially constant, wave energy amplifying means including a tunableelement which determines the frequency of the wave energ amplified, apair of rectifier tubes having input and output electrodes, at frequencydiscriminating circuit connecting the input electrodes of saidrcctifiers in a differential circuit, pedances connecting the outputelectrodes of said rectifiers in a balanced differential circuit, meansfor impressing said wave energy on said frequency discriminatingcircuit, whereby sub stantially equal differential voltages are producedin said impedanccs by rectified wave energy when said wave energyimpressed on said frequency discriminating circuit is of the proper meanfrequency and unequal difierential voltages are produced in saidimpedances when the frequency of said wave energy deviates from saidproper frequency, a pair of electron discharge amplifier tubes havinginput and output electrodes, means coupling said input electrodesdiiferentially to said impedances, additional impedances coupling theoutput electrodes of said tubes in a differential circuit, means forbalancing said last named differential circuit whereby the differentialvoltages in said last named impedances are substantially balanced whenthe wave energy impressed on said frequency discriminator circuit is ofthe the other direction when direct current flows through the other ofsaid devices, energizing circuits for said motor, normally open contactsin said circuits, and means actuated by current flow in each of saiduni-lateral devices for closing a different one of said contacts.

BERTRAM TREVOR.

